Field of the Invention
Embodiments of the present application relates to the art of a power transmission unit such as a transmission, a transaxle and a differential unit, and especially to a hydraulic cooling system for a power transmission unit.
Discussion of the Related Art
JP-A-2015-034581 describes an oil supply device of a hybrid vehicle having a first motor and a second motor. According to the teachings of JP-A-2015-034581, oil in a transmission is delivered to an oil sump passage by a rotation of a final gear to lubricate an oil requiring site. In the transmission, the oil is also delivered to an oil introduction passage by rotations of an output gear and a counter gear meshing with each other through a guide. Thus, the oil requiring site may be lubricated not only by the oil delivered through the oil sump passage but also by the oil delivered through the oil introduction passage. According to the teachings of JP-A-2015-034581, therefore, an ample amount of the oil can be applied to the oil requiring portion by the rotations of the output gear and the counter gear even if a rotational speed of the final gear is not high enough.
According to the teachings of JP-A-2015-034581, however, temperature of the oil thus circulating in the transmission may be raised gradually in spite of radiating heat through an oil pan. Especially, in the hybrid vehicle, oil temperature may be raised not only by friction heat resulting from rotations of the gears and shafts, but also by Joule heat resulting from operating the motors. For this reason, the transmission may not be cooled sufficiently by such splash lubrication of the oil.
In order to enhance cooling performance of the oil, an oil cooler may be arranged in the hybrid vehicle. In this case, however, a structure of the oil passage may be modified in such a manner as to deliver the oil cooled by the oil cooler effectively to a cooling site while reducing a cost.